TextureCacheBase: Fix possible crash on shutdown with deferred EFB copies

Source/Core/VideoCommon/TextureCacheBase.cpp

@@ -96,6 +96,9 @@ TextureCacheBase::TextureCacheBase()
 
 TextureCacheBase::~TextureCacheBase()
 {
+  // Clear pending EFB copies first, so we don't try to flush them.
+  m_pending_efb_copies.clear();
+
   HiresTexture::Shutdown();
   Invalidate();
   Common::FreeAlignedMemory(temp);